Electrode structure for gas discharge devices



July 25, 1950 CARNE 2,516,675

ELECTRODE STRUCTURE FOR GAS DISCHARGE DEVICES Filed Feb. 5, 1947 INVENTQR 5159410 6, 040v:-

ORNEY Patented July 25, 1950 ELECTRODE STRUCTURE I AR EVlQE f Gerald; 6.1; Game, .Rockaway', N. J assignor to. ad o-;.& Qr a ion .o ericas.) corporation of De aware invention relates to electron discharge -de-- vices of the gas-j type and moregrid controlledgas t1'1loes.-

One form of gas tube' employingagaseous atmosphere, preferably argon or Xenon, consists ofna thermionic cathode; an anode and a control particularly to electrode mountedzinalignment between the two.

Aimetal :shield encloses-the threeelectrodes and has a -portionforming-a screen grid between the anode and control electrode. When an A. C.-

potential difference is=imposed between the anode andi-cathode or the tubea discharge takes place uponthe--half cyclewhen the anode is positive. H

Applying a-negative bias below-a critical value upon the controhgrid willpreventthe initiation ofxsucha-discharge; The controlgrid bias may be shift-ed in-apositive direction above this critical; voltage value by the application of a signal voltage so as to permit thetube to conduct In some applications oi these tubes it is desirable-to control-a power circuit by a small input The use of a high grid resistance has presentedv some seriousproblems; When a large impedance isplacedrinethe-grid circuit'any small flow ef current inthecircuit'will' cause a-shift-of the voltageiiponithe grid-toward the positive by an appreciable amount} Anundesired flow of -cur rent :in thegridcircuitmay be initiated by various conditionswithin the tube, the eiT'ects of which are-accumulative.- Such an uncontrolled shift-in grid-voltage causesra loss of control of tube operation by-permittingan unscheduled discharge of theetubei Thisshifteoithegridwoltage may in a simple ca se be-..counteracted :by. increasing the; grid'mias-voltaged However. this is:-not always feasible;;.. Iniotheri cases theshift is unpredictable nd; erratic.

in. the gridgcontrolcharacteristic;

Ittis: therefore an. object-of my. invention to providera gas-.controltube of improved operation.

It is also an object, .of myqinvention to provide;- a; grid --.co ntrolled gas discharge tube having a;

constantgrid voltage breakdown characteristic.

The novel featuresl which .I believe to be. characteristic otmm-invention are set forth with -par-,

ticularity in the appended claims, butthev invene.

ck-i sel will beste erunderstood;byi reference Intsuchwlater-cases it is'more-expedient to eliminate the causes of such a shift 5s ;.is aligned in a parallel sense with both t cathode of the tube will initiate answer 'electro 2 to the. following description taken inconnection; with the;accompanying drawing, inwhich'i Fig 1' is a .partial 'sectiona-l viewof a gas discharge tube accordingtolmy inventiony Fig.- 2 ;is;a cross-sectional view along the sec-' tional lineAiA ofiFig. 1"; and Fig. 3 he .circuit diagram of-an applicationutilizingithe:..tube0f Figs. land-2; w Thev gas. tube disclosed in iFig. 1 has 1 an envelope lfllclosed' atithe; bottom by. a glass ste portion, not shown. Projectingup from thest'r portion is :a glass press-i 2 upon-which are inount a. plurality of. electrodes: Aycathfodesleeve supported :from press; I2 is thermionically' heated; by a filament I 5. The outer surface of the'cathod" sleeve ll this lcoveredsina wellknown manner wit an electron emissive c'oating The'anode T6 is along narrowwplate mounted-axiallyparallel'to 1 the cathode-.-.sleeve T .l 4: Btween the cathode] 4f; andsthe. anode l 6 is mounted a control grid l8 in Y theform ofan elongated metal ribbonloopj Asshown in- Figs- 1 and 'Z th e control grid i8 i 'formed with flat sides enclosi'nga longina rrowj aperture 1 I. This. aperture fl 1 of the: control grid 1', he cathiofl fleiel shield com v cylinder 1 4 and the anode plate l6? surrounding the three electrodes isa prising.astubularishieldportion z'ggnelcsmg' ei; trodes-14;] 6 and I 8 and'shieldplates damp closingathe open ends=0f tubular shield' 2G. Micai spacers 22 and 2 l -hold'in alignment'arid'insulated from each other the cathode cylinder Hand control grid 18. The space within the tubul r shieldZllfiisdivided into av portions-b'yia pai tition 26- which is fixed to the snie dzn and" Saidrates thetanode: [B from controllgr id I 11am; cathode; l4. Anaperture 28 is provided'fin par tion 26' in.such awaythat-catliode' I 4 issh'ielld; d"

from. all anode: field effects exceptthoselliries pf' a force passing through apertureffll "Ihfat; ,"the. partition: 26: provides an efictive shield grd structur between the anode andl cath odelelegg" trodes Adiiference-of potential between the anode and x;

, V lr H from the cathodejdto the anode Thistle tron emission ionizesthegas ofjthetub e thereby establishing an-arc discharge vbetween the anode and cathode. The ionization path and'consef rquently .thearc discharge are confined tyne metal. shie1ds |9;.-.20= and 2 ltothe s'pace between t the anode; and cathode electrodes This WeILd-I finedzarc discharge also passes essntially throiighl the-controlrgrid.aperture' H. T When a sufiiciently negative bias. is .placed uponn the control gridf 3 breakdown of the tube and ionization of the arc discharge is prevented. A suiiiciently large positive voltage applied to the grid will overcome the negative control bias and permit the ga arc discharge to take place. It is characteristic of gas tubes of this type that once this are discharge is established the grid loses control and is unable to stop the discharge. However, a negatively biased control grid will regain control of the tube when the anode voltage is removed or reversed in polarity.

Fig. 3 discloses a relay circuit in which a relay 56 is energized by an increase in light falling upon a photoelectric tube 50. In this circuit electrical energy is derived from a source of alternating current applied to circuit terminals 32. A positive voltage is supplied to the anode I6 of the gas control tube I during every other half cycle of line voltage when the circuit conductor 36 of the alternating current line is positive. During the half cycles when the anode l6 of control tube I0 is positive, control grid I8 receives a negative bias through the resistance 44 which is connected to the negative conductor 38 of the circuit. Contact46 on the resistance '44 is made adjustable so as to vary the negative value of this control grid bias. Resistance 44 is part of a shunt 40 which includes a resistance 42 between the positive portion 36 and the negative portion 38 of the A. C. circuit. In this particular application, resistance 42 may be around 20,000 ohms, while resistance 44 may be around 1,000 ohms. The cathode l4 of the gas control tube ii) is connected to this shunt 40 at 43 a point between the resistance 42 and 44. With this arrangement, the IR, drop across resistance 42 determines the difference in potential between the cathode i4 and the anode H5. The anode 52 of a phototube 50 is connected directly to the positive portion 36 of the A; C. circuit. y Aphotoelectron emitting cathode 54 of the phototube 58 is connected directly to the negatively biased grid circuit 45. A large resistance 48- preferably of from oneto megohms is inserted into the control grid circuit 45. In the plate circuit of the control tube ID, a relay 56 is connected in series and is adapted to be operated whenever there is a current flow or arc discharge between the cathode I4 and the anode l6 of tube ill. The filament for heating the cathode l4 of tube In derives its source of energy from the secondary of a transformer 58 whose primary is connected in the alternating current circuit.

This circuit arrangement of Fig. 3 utilizes the gas tube ID as a control for the operation of relay 56. The conduction of tube i0 and thereby the energization of relay 56 is governed by an increase of light impinging upon the photoelectron emitting cathode 54 of the phototube 50. Contact of the control grid circuit 45 may be adjusted so that as long as illumination upon the cathode 54 is less than a certain value the control grid I8 will remain sufl'iciently negative to prevent conduction of tube It. When the illumination rises above this predetermined value the IR dropacross resistance 48 reduces the negative potential of the control grid i8 above the critical cut-off value and the tube 10 then conducts to operate relay 56.

In order to retain proper control of the gas tube In, in such applications as shown in Fig. 3, the negative bias of the control grid 18 should remain constant. This is possible as long as the negative charge on control electrode [8 is not diminished in any undesired manner. However, the

negative value of the charge on grid I8 may be reduced by neutralization due to the flow of positive gas ions to the control electrode and due to escape or leakage of electrons from the control electrode. The presence of the high resistance 48 in the grid circuit 45 prevents the maintenance of the desired negative grid bias if the factors neutralizing the grid are sufliciently large. In this latter case there would result a shift of the grid potential toward the positive and a consequent loss of grid control. This phenomenon of grid neutralization may be produced by a plurality of factors. Some of these factors are well recognized and have been eliminated or counteracted by various Ways. Ionization of the gas in control tube it, may occur between exposed leads within the tube or due to electronic emission between grid 18 and anode l6. Electron leakage from the control grid l8 may occur because of high difference of potential between this electrode and the anode I6. Electron emission from the grid l8 may also occur from its bombardment by positive gas ions and as its temperature increases during tube operation.

The particular problem that I am concerned with, in this case, is the reduction of the positive grid shift due to the unscheduled ionization of the gas within the control tube l0.

Emission from control grid [8 can be reduced by lowering the grid operating temperature or in increasing the work function of grid electrode 18. Several experiments were tried to reduce control grid emission such as gold plating the control electrode l6; using for grid la a magnonickel wire which has a low electron emission; and the provision of heat radiating fins upon grid I8. I found that all of these expediencies helped to reduce the value of the positive grid shift only by approximately one-third. The situation was improved but in no way solved. I felt that it was quite possible that electron emission could be taking place from'the shield grid 25 or from some other portion of the shield 20. However, this was not obvious as several attempts which I made to measure a current flow to the shield failed.

In this type of gas control tube the shield grid 26 is much closer to the anode and has an unobstructed path thereto. In contrast, the control grid is is entirely shielded from the anode i 6. I felt that with these conditions it was conceivably possible that a much less electron emission from the shield electrode 26 than that from the control grid 18 would produce sufficient gas ionization for'tube breakdown. After appropriate tests I estimated that if one microampere of control grid emission was required for tube failure due to misconduction, then approximately 0.01

microampere of shield grid emission would pro-' duce a similar tube failure. Basing my work on these tests and conclusions, I have developed a shield for this tube which has reduced the grid bias shift to an amount where it is no longer a factor.

My invention is a means to reduce shift in control voltage of the control grid by reducing shield emission through lowering the temperature of the shield 20 and of the space enclosed by this shield. This means is a provision of a dull roughened surface 23 on the outside of the grid 20. This great increase in surface area due to the roughening 23 results in a large increase in heat radiation by the shield 20.

The art teaches many methods and types of This undesired ionization is due principally to deleterious emis- 'sion from various parts within the envelope.

roughened electrode surfaces. Several possible procedures such as sand-blasting, acid etching and electropolishing were tried but they did not give the desired results. In some instances an insufficient improvement in grid shift was obtained while other methods proved impractical from a standpoint of tube manufacturing. The problem was further complicated by the fact that the tube with which I was working was a gas tube in which the gas pressure was critical. Thus, a method of carbonizing the shield was found to change the gas pressure within the tube due to to the carbon absorbing the gas.

I discovered that the best method of reducing the shift in the grid characteristic was by spraying the shield 20 with a coating of nickel oxide which when reduced by hydrogen firing presented a dull rough nickel surface 23. The nickel oxide was mixed with a nitrocellulose binder. This coating mixture can be applied to the shield 20 by several ways such as painting, spraying, or dragging. The coated shield is air dried and then placed in a hydrogen furnace for approximately 10 minutes at 1000 C. During this hydrogen firing the nickel oxide is reduced to finely divided nickel particles. At this furnace temperature, the nitrocellulose binder is burned on" and the nickel particles sintered to the surface of the metal shield 20. The time of firing and the furnace temperatures are not critical as the same reaction can take place at lower temperatures such as 700 over a longer period of time.

This method of providing a roughened nickel coating 23 to the shield electrode 20 has resulted in reducing the grid voltage shift from approximately volts to less than 0.5 volt, a point where the grid shift is not a problem. There may be several explanations for this improvement of tube operation but undoubtedly the greatest contributing factor is reduction of the operating temperature within the interelectrode space enclosed by the tube shield 20. It is apparent that the increased surface of radiation of the shield, as well as its dull coloration, lowers the temperature of grid 26 and shield 20 to a point where a gas ionizing electron emission from the shield grid 26 or any part of the shield 20 to anode I 6 is at a minimum.

While certain specific embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What I claim as new is:

1. An electron discharge device comprising a sealed envelope, a gaseous medium within said envelope, an anode electrode mounted within said envelope, an electron emitting electrode spaced within said envelope from said anode electrode, a non-emitting control electrode mounted between said anode and said electron emitting electrodes, a tubular metal shield within said envelope enclosing all of said electrodes, said shield having an apertured portion extending between said anode and said control electrode, said shield having a rough coating of nickel metal particles sintered to the outer surface thereof to minimize electron emission between any portion of said shield and said anode electrode.

2. A gaseous discharge device comprising a sealed envelope, an anode mounted within said envelope, an electron emitting cathode spaced within said envelope from said anode, a shield electrode within said envelope adapted to be negatively biased relative to said anode and having a portion extending between said anode and cathode electrodes, a rough coating of nickel metal particles sintered to a portion of the surface of said shield electrode to minimize electron emission between said shield and said anode electrode.

3. A shield assembly adapted to be disposed at a negative bias within the sealed envelope of a gaseous electron discharge device, comprising a tubular metal body portion for enclosing the electrodes of said tube and a metal plate closing each end of said tubular body to confine the gaseous discharge of the tube in the space within said tubular body portion, a rough coating of nickel metal particles sintered to the outer surface of said cylindrical body portion to minimize electron emission from said shield.

GERALD G. CARNE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,794,315 Mullaney Feb. 24, 1931 2,040,883 Solomon May 19, 1936 2,160,086 Ronci et al May 30, 1939 2,203,639 Smith June 4, 1940 2,431,237 Freeman Nov. 18, 1947 

